1. Field of the Invention
The present invention relates generally to fabrication of metal oxide semiconductor field effect transistors (MOSFETs), and, more particularly, to MOSFETs that achieve improved carrier mobility through the incorporation of strained silicon.
2. Related Technology
MOSFETs are a common component of integrated circuits (ICs). FIG. 1 shows a cross sectional view of a conventional MOSFET device. The MOSFET is fabricated on a silicon substrate 10 within an active region bounded by shallow trench isolations 12 that electrically isolate the active region of the MOSFET from other IC components fabricated on the substrate 10.
The MOSFET is comprised of a gate 14 that is separated from a channel region 16 in the substrate 10 by a thin first gate insulator 18 such as silicon oxide or silicon oxynitride. A gate voltage applied to the gate 14 controls the availability of carriers in the channel region 16. To minimize the resistance of the gate 14, the gate 14 is typically formed of a doped semiconductor material such as polysilicon.
The source and drain of the MOSFET comprise deep source and drain regions 20 formed on opposing sides of the channel region 16. The deep source and drain regions 20 are implanted by ion implantation subsequent to the formation of a spacer 22 around the gate 14, which serves as a mask during implantation to define the lateral positions of the deep source and drain regions 20 relative to the channel region 16.
Source and drain silicides 24 are formed on the deep source and drain regions 20 and are comprised of a compound comprising the substrate semiconductor material and a metal such as cobalt (Co) or nickel (Ni) to reduce contact resistance to the deep source and drain regions 20. The deep source and drain regions 20 are formed deeply enough to extend beyond the depth to which the source and drain silicides 24 are formed. The gate 14 likewise has a silicide 26 formed on its upper surface. A gate structure comprising a polysilicon material and an overlying silicide is sometimes referred to as a polycide gate.
The source and drain of the MOSFET further comprise shallow source and drain extensions 28. As dimensions of the MOSFET are reduced, short channel effects resulting from the small distance between the source and drain cause degradation of MOSFET performance. The use of shallow source and drain extensions 28 rather than deep source and drain regions near the ends of the channel 16 helps to reduce short channel effects. The shallow source and drain extensions 28 are implanted after the formation of a thin spacer 30 around the gate 14 and prior to the formation of the spacer 22, and the gate 14 and thin spacer 30 act as an implantation mask to define the lateral position of the shallow source and drain extensions 28 relative to the channel region 16. Diffusion during subsequent annealing causes the shallow source and drain extensions 28 to extend slightly beneath the gate 14.
One option for increasing the performance of MOSFETs is to enhance the carrier mobility of the MOSFET semiconductor material so as to reduce resistance and power consumption and to increase drive current, frequency response and operating speed. A method of enhancing carrier mobility that has become a focus of recent attention is the use of silicon material to which a tensile strain is applied. “Strained” silicon may be formed by growing a layer of silicon on a silicon germanium substrate. The silicon germanium lattice is more widely spaced on average than a pure silicon lattice because of the presence of the larger germanium atoms in the lattice. since the atoms of the silicon lattice align with the more widely spread silicon germanium lattice, a tensile strain is created in the silicon layer. The silicon atoms are essentially pulled apart from one another. The amount of tensile strain applied to the silicon lattice increases with the proportion of germanium in the silicon germanium lattice.
Relaxed silicon has six equal valence bands. The application of tensile strain to the silicon lattice causes four of the valence bands to increase in energy and two of the valence bands to decrease in energy. As a result of quantum effects, electrons effectively weigh 30 percent less when passing through the lower energy bands. Thus the lower energy bands offer less resistance to electron flow. In addition, electrons encounter less vibrational energy from the nucleus of the silicon atom, which causes them to scatter at a rate of 500 to 1000 times less than in relaxed silicon. As a result, carrier mobility is dramatically increased in strained silicon as compared to relaxed silicon, offering a potential increase in mobility of 80% or more for electrons and 20% or more for holes. The increase in mobility has been found to persist for current fields of up to 1.5 megavolts/centimeter. These factors are believed to enable a device speed increase of 35% without further reduction of device size, or a 25% reduction in power consumption without a reduction in performance.
An example of a MOSFET using a strained silicon layer is shown in FIG. 2. The MOSFET is fabricated on a substrate comprising a silicon germanium layer 32 grown on a silicon layer 10. An epitaxial layer of strained silicon 34 is grown on the silicon germanium layer 32. The MOSFET uses conventional MOSFET structures including deep source and drain regions 20, shallow source and drain extensions 28, a gate oxide layer 18, a gate 14 surrounded by spacers 30, 22, source and drain silicides 24, a gate silicide 26, and shallow trench isolations 12. The channel region of the MOSFET includes the strained silicon material, which provides enhanced carrier mobility between the source and drain.
One detrimental property of strained silicon MOSFETs of the type shown in FIG. 2 is that their thermal conductivity is significantly less than that of standard MOSFETs. Heat generated in the active regions of standard MOSFETs is conducted away from the active regions through the silicon substrate, which has relatively good thermal conductivity of 1.5 W/cm-C. Although the active regions are surrounded by shallow trench isolations that are filled with silicon oxide, which has a very poor thermal conductivity of 0.014 W/cm-C, the vertical path from the active regions to the bulk silicon material beneath substrate is sufficient to dissipate thermal energy generated in the active regions. In contrast, the silicon germanium layer used in strained silicon devices is a relatively poor conductor of heat, having a thermal conductivity that is approximately 0.1 W/cm-C for a silicon germanium layer having 20% germanium, or approximately one-fifteenth the thermal conductivity of silicon. As a result, the dissipation of thermal energy to the bulk silicon material is impeded by the silicon germanium layer, and significant self-heating problems can arise. Self-heating is known to degrade the I-V characteristics of the MOSFET, such that a reduced source-drain current Ids is produced for a given source-drain voltage Vds.
Therefore the advantages achieved by incorporating strained silicon into MOSFET designs are partly offset by the disadvantages resulting from the use of a silicon germanium substrate.